1. Field of Invention
This invention relates to a printing ink, an indicia-bearing laminated label for permanent fusion to polyolefin surfaces, and in particular to a method for in-mold indicia marking of rotationally molded products.
2. Background of the Invention
The application of indicia to polyolefin products has evolved from adhesive labels or overlays to in-mold applications, with each advance seeking greater ease of application and durability of the indicia on the products. Adhesive labels have never achieved permanent marking of polyolefin products, particularly polyethylene products, which have surfaces that resist coatings and adhesives.
An early attempt to achieve permanent marking of polyolefin products is described in U.S. Pat. No. 4,252,762 in which a transfer having an indicia array screen-printed with an oil suspension of polyethylene and pigments was applied to the interior surface of a rotational mold. During the molding cycle, the indicia array melted and fused into the rotationally molded product, resulting in the first successful, permanent fusion of indicia into the surface of a molded polyolefin product.
Successive improvements on in-mold fusion of indicia into rotationally molded polyolefin products concentrated on improvements of the indicia medium used for printing the transfer, e.g., U.S. Pat. No. 4,519,972 discloses use of pigmented wax printing media.
The transfer application technique was applied to indicia marking of preformed molded polyolefin products in U.S. Pat. No. 5,840,142. All of these attempts resulted in successful fusion of indicia into the outer surface of molded polyolefin products.
In-mold labeling of polyolefin products has also been the subject of patents in which adhesive labels are applied to the surfaces of molds used in blow molding processes, e.g., see U.S. Pat. Nos. 5,925,208 and 6,991,261. Because of the nature of blow molding process, the labels do not melt and fuse into the molded product, and must be adhesively bonded, resulting in a nonpermanent marking of the molded product.
It has been suggested in U.S. Pat. No. 5,498,307 that a label of indicia printed on extruded polyethylene film can be used for in-mold application to rotationally molded products. Labels printed on preformed polyolefin films, however, are invariably oriented such that the indicia array is distorted when the film is melted under the conditions of rotational molding.